The dual epidemics of HIV and HCV infections have been reported in many parts of the world. The two viruses share common transmission routes but with different efficiency. While mounting evidence has shown infection of HIV is influenced by both individual and social network level factors, relatively few studies have intensively examined factors contributing to HCV mono-infection and its co-infection with HIV. The goal of this study is to investigate the role of personal network in HCV mono-infections and HIV/HCV co-infections among injection drug users (IDUs). In particular, the following specific aims are to be addressed: 1) to estimate the prevalence of HCV mono-infection and HIV/HCV co-infection in a large cohort of IDUs; 2) to examine personal network characteristics as risk factors for HCV mono-infection and HIV/HCV co-infection among IDUs; and 3) to examine the association between HCV-specific injection risk behaviors and HCV specific sexual risk behaviors among IDUs. This cross-sectional study is nested in an HIV prevention intervention study in Chennai, India. A total of 600 inner city IDUs will be recruited through targeted outreach in areas of high drug activity in Chennai. Information from this study will provide insight into HCV transmission dynamics in the era of HIV. Such knowledge may direct appropriate HIV/HCV combined intervention to prevent disease transmission, and as a consequence, reduce the socioeconomic burden from HCV liver diseases and possibly HIV disease progression.